1. Technical Field
The present invention relates to a resonator element that excites in a thickness-shear vibration mode, a resonator, an electronic device, an electronic apparatus, and a method of manufacturing a resonator element.
2. Related Art
A quartz crystal resonator, in which an AT cut quartz crystal resonator element whose main vibration is excited in a thickness-shear vibration mode is used, is used in various fields, such as a piezoelectric oscillator and an electronic apparatus, since the quartz crystal resonator is suitable for miniaturization and higher frequencies and exhibits a three-dimensional curve with an excellent frequency-temperature characteristic. In particular, in recent years, as the processing speed of transmission and communication equipment or OA equipment increases or the amount of communication data or the progressing amount increases, request for higher frequencies is growing for the quartz crystal resonator as a reference frequency signal source used in the equipment. In the AT cut quartz crystal resonator element that excites in a thickness-shear vibration mode, since there is an inverse relationship between the frequency and the plate thickness of a vibrating portion, the plate thickness of the vibrating portion is generally set to be small in order to obtain higher frequencies. For this reason, in order to obtain higher frequencies, a thin vibrating portion is provided at the bottom of a recessed portion, which is formed in the middle of the quartz crystal substrate by dry etching or wet etching from one surface side or both surface sides, and main electrodes for exciting the vibrating portion are formed on the top and bottom main surfaces of the vibrating portion and lead electrodes extending in the outer peripheral direction are formed on both surfaces of the quartz crystal substrate.
However, there is a relationship in which the equivalent series inductance L1 of the equivalent circuit constant increases as the degree of parallelism of the vibrating portion of the quartz crystal resonator element degrades. Accordingly, in the case of a high-frequency quartz crystal resonator element having a resonance frequency of 100 MHz or higher, the plate thickness of the vibrating portion located at the bottom of the recessed portion becomes very small (16.7 μm or less). As a result, since the degree of degradation of the degree of parallelism with respect to the plate thickness of the vibrating portion becomes large, an influence on the equivalent series inductance L1 also becomes very large. For this reason, even with the degree of parallelism of the vibrating portion that has hardly caused a problem in a low-frequency quartz crystal resonator element, the equivalent series inductance L1 becomes larger than the design value and the variation is also increased in a high-frequency quartz crystal resonator element. Therefore, there has been a problem in that the yield of the quartz crystal resonator element is significantly reduced.
JP-A-2005-72710 discloses a manufacturing method for improving the degree of parallelism by dividing a vibrating portion formed in a recessed portion into three regions, measuring the plate thickness of each region, and then performing local etching using a resist as a protective film.
In the manufacturing method disclosed in JP-A-2005-72710, however, when local etching of a plurality of vibrating portions formed in a large quartz crystal substrate is simultaneously performed, a photomask corresponding to the plate thickness of each vibrating portion is required in order to form a protective film. Accordingly, since it is necessary to prepare a photomask corresponding to the plate thickness of a vibrating portion for each large quartz crystal substrate at the time of mass production, the manufacturing cost is increased due to the cost for generating the photomask and the like. As a result, cost reduction has been in demand. In addition, in the case of a high-frequency quartz crystal resonator element having a resonance frequency of 200 MHz or higher, the electrode also becomes small as the thickness of the vibrating portion decreases. Therefore, since the degree of parallelism of a portion in which the electrode is formed cannot be improved just by dividing the vibrating portion into three regions, there has been a problem in that the variation in the equivalent series inductance L1 or the equivalent series capacitance C1 is large and unwanted spurious is caused.